Horizontal Cone Penetration Testing in Sand

P.P. Rahardjo & T. Lunne (eds.) In Situ Measurements and Case Histories, Bandung, Indonesia, 2001, pp. 649654.

Horizontal Cone Penetration Testing in Sand

W. BroereGeotechnical Laboratory, Delft University of Technology, The Netherlands

A.F. van TolRotterdam Public Works, The NetherlandsGeotechnical Laboratory, Delft University of Technology, The Netherlands

ABSTRACT: The cone resistance and sleeve friction measured when a penetration test is executed in thehorizontal direction differ from those obtained when sounding in the traditional vertical direction. In order toquantify the differences a number of tests has been executed in a 2m diameter rigid wall calibration chamberon sands with different densities and different gradations. These tests have shown that the difference betweenhorizontal and vertical cone resistance is greatest for medium dense sands, but show little dependancy on thegradation of the sand. The ratio of horizontal over vertical sleeve friction on the other hand shows little variationfor different densities, but is strongly dependant on the coarseness of the sand.

1 INTRODUCTION

The cone penetration test (CPT) is traditionally ex-ecuted in a downward vertical direction in order to ob-tain information about the soil properties and stratifica-tion. The interpretation of the obtained measurementsis subsequently made using analytical and empiricalmodels, as well as experience, which all implicitely orexplicitely use the assumption that the penetration dir-ection is vertical. This is in general not regarded as aproblem and the measures taken to limit the deflectionfrom the vertical during penetration, or to measure thisdeflection and correct for it, do not originate from thepossible problem in the interpretation of cone resist-ance. Those measures are taken because the deflectionmight lead to unacceptable errors in the depth regis-tration and thereby the stratigraphy.

The introduction of mechanised tunnel boring in softand strongly heterogeneous soils, as found in manydelta-areas over the world, has given rise to a needfor more detailed information along the alignment ofthe tunnel. At the same time the possibility has beencreated to obtain this information using cone penetra-tion tests originating from the tunnel boring machinein a horizontal forward direction, and it has been re-cognised that this could complement the informationgained from vertical CPTs. Initiatives have also beenunfurled to execute horizontal CPTs through retain-ing walls of building pits to investigate the soil con-ditions below adjacent buildings, or through existingrailway embankments and land fills, without disturb-ing the topside actvities.

Now the equipment used to perform a vertical CPTcan be easily converted to execute a horizontal cone

penetration test (HCPT). The interpretation is not soeasily converted however. It has been recognised byHoulsby & Hitchman (1988) that the effective hori-zontal stress h is the controlling stress component intraditional CPT calibration chamber tests on sand. Invertical CPT this is the stress that acts in the planeperpendicalur to the penetration direction, and in thisplane can be considered a radially uniform stress com-ponent. In the case of HCPT the stress state perpen-dicular to the cone is not radially uniform, as it willvary between h and the effective vertical stress v.Combined with the fact that most soils have been de-posited in a layerwise manner, it is to be expected thatthe measurements obtained from HCPT differ fromthose in vertical CPT.

1.1 HCPT experiences

This difference between horizontal and vertical CPThas already been observed in calibration chamber testsas well as in field tests. The calibration chamber testsby Broere & van Tol (1998) were performed in a uni-formly distributed fine sand and showed a horizontalcone resistance qHc greater than the vertical cone res-istance qVc at the same depth. The horizontal coneresistance was on average 1.2 times the vertical formedium densified sands, whereas the ratio was closerto one for very loose or dense sands. The horizontalfriction ratio on the other hand was clearly lower thanthe vertical friction ratio, but showed little dependancyon the density of the sand.

A similar image can be derived from the field meas-urements by van Deen et al. (1999). The main partof the tests was executed in soft clay and peat layers

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instead of sands, but shows similar trends. The hori-zontal cone resistance measured was greater than thevertical, even up to three times greater in clay. Andagain the horizontal friction ratio was on average lowerthan the vertical. And although on one hand these res-ults strengthen the results from the calibration chambertests, they also indicate that there may be an influenceof the soil type.

1.2 Reseach aims

It has been recognised early on that the measurementsfrom CPT in sand depend not only on the stress leveland (relative) density, but amongst others also on thegrain size distribution of the material and the amountof fines present. See for example the overview of soilclassification charts presented by Lunne et al. (1997)or the comparison of numerous calibration chambertests by Jamiolkowski (1988). It is also very likelythat HCPT measurements show a similar dependancyon the gradation of sand, but this dependancy is notnecessarily exactly equal. Therefore the ratio of ho-rizontal over vertical cone resistance (qH/Vc ) and theratio of horizontal over vertical sleeve friction (fH/VS )may also depend on the soil type.

In order to investigate this behaviour a number oftests has been executed in the large rigid wall cal-ibration chamber of Delft University of Technology(DUT). These tests have been limited to sand samples,as the preparation of large, homogeneous, cohesivesamples in a calibration chamber is difficult and time-consuming. Three different sands have been used,which have been prepared at various densities. Withineach sample two vertical and one horizontal CPT hasbeen made and the measurements at the depth of theHCPTh have been compared.

2 THE CALIBRATION CHAMBER

The DUT calibration chamber is a 2m diameter rigidwall calibration chamber, as sketched in figure 1. Atthe bottom of the tank a system of filter drains connec-ted to a pumping system is embedded in the sand. Thissystem allows fluidisation of the sand bed within thetank. A couple of vibrators affixed to the sides of thetank can be used to vibrate the entire tank and therebydensify the sand. To prepare a sample the sand is firstfluidised and, when fully liquified, the water is allowedto drain. The vibrators are then used for a period of 0to 8 minutes, dependant on the relative densify of thesand that is required. After that the remaining wateris allowed to drain and the density of the sample isdetermined by measuring the top level of the sand.

The preparation method used, fluidisation instead ofthe more common pluviation, is one of the main dif-ferences between the DUT calibration chamber andmost other chambers. The great advantage of this

1900

3230

780

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z top

Vibrator

Upper sounding lock

Lower sounding lock

Fluidisation system

Figure 1. DUT calibration chamber

method is that the preparation of the sample is solabour-extensive; a new sample can be prepared eachday, requiring only one man-hour actual work duringthis period. The disadvantages are that the samplesare somewhat less homogeneous over the height of thesample than pluviated samples, that due to the fluidisa-tion process fines may slowly be washed out of the sandused, and that the obtained sample is unsaturated butnot dry, and that the level of saturation depends on thepermeability of the sand and the draining time. Thislast problem can be circumvented completely, as thechamber allows the testing of fully saturated samplesas well, but this option has not been used in this testseries.

The other major difference is that the DUT chamberis a rigid wall calibration chamber, meaning that thelateral boundaries are stiff and disallow any deforma-tion of the sample. This in contrast to the often usedflexible wall chambers, where a pressurised membraneis used at the lateral boundaries to keep a constant lat-eral pressure. As is common the lower boundary is thestiff chamber floor, whereas at the top an overburdenload could be applied. The sample is large enoughhowever that reasonable stress levels are reached atthe depth of the HCPT opening without an overbur-den. The result of this combination of boundaries isthat the DUT chamber falls somewhere between BC2& 3 as given by Parkin (1988) as:

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0.050 0.100 0.200 0.500 1.0 2.0sieve aperture (mm)

pass

edsie

ve

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fine middle coarsesand

1 2 3

Figure 2. Sieve curves for all sands

Table 1. Minimal and maximal densitiesSand emin emax1 0.498 0.8012 0.454 0.7493 0.431 0.746

B1 v, h constantB2 v = h = 0B3 v constant, h = 0B4 h constant, v = 0

This implies according to Salgado (1998) that the coneresistance qc measured in the tank is higher than wouldbe measured in the same conditions in the field. Atthe chamber diameter-to-cone diameter ratio Rd = 56this is only a few percent however and the effect canbe neglected.

A further special feature of the tank is of coursethe presence of a lock on the side wall of the tank,as sketched in figure 1. This lock allows a horizontalpenetration to be made using a standard 35mm cone.

3 THE SANDS

Three different sands have been used, which differ intheir gradation and coarseness. The first sand is a uni-formely distributed fine sand of alluvial origin. Thisbatch of sand had been used before in the same cal-ibration chamber, as a result of which most fines hadbeen washed out. The second and third sand type hasbeen obtained by mixing this alluvial sand in differ-ent proportions with a commercially available coarseriver sand, which had been washed to remove all fines.The sieve curves of the resulting sands are shown infigure 2.

For these sands the minimal and maximal densit-ies have been obtained by pouring dry sand through afunnel respectively vibrating and compacting a moistsample for an extended period of time. The resultingemin and emax are listed in table 1.

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pass

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fine middle coarsesand

1.0m depth1.5m depth2.0m depth

Figure 3. Sieve curves for sand 2 at different depths; influ-ence of segregation

During the preparation of sands 2 and 3 a certainamount of segregation is observed due to the fluidisa-tion process, as the finer particles tend to float upwards.As a result the sand in which the HCPTs are made issomewhat coarser than would be derived from figure 2.This can be seen in figure 3, where the sieve curvesfrom samples of sand 2 at three different depths areplotted. The uppermost layer clearly differs from thelower layers, although the lower layers do not differstrongly from the overall sieve curve. The correlationbetween vertical and horizontal CPT measurements ismade for the 2m depth level only and the sand in thisregion does not show any local segregation influences.Sand 3 exhibits the same behaviour (not shown), al-though the differences between the overall gradationcurve and those of the lower segregated layers are evensmaller than for sand 2. Sand 1 on the other hand is souniform that no segregation effects can be observed atall over the height of the tank.

The overall effect of this segregation is that the rel-ative density Dr calculated for samples of sand 1 hasan estimated error of 2%, whereas for sands 2 and 3this error is closer to 5%. This error is of the samemagnitude as in most calibration chamber tests andnot considered a problem.

4 OVERVIEW OF TEST SERIES

For each of the three sands 10 samples have been pre-pared, with different vibration times and as a resultdifferent relative densities. Within each sample twovertical penetrations were made followed by one hori-zontal, at locations as sketched in figure 4. Althoughsketched here in the same vertical plane, the path ofthe horizontal penetration lies in a plane at a 10 anglewith the plane of the vertical penetrations. As a resultthe path of the horizontal penetration passes the pathsof the vertical penetrations at approximately 10cm.

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Figure 4. Locations of vertical and horizontal CPTs

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0 2 4 6 8 10 12 14 16 18 20 qVc (MPa)02468101214161820 qHc (MPa)

Figure 5. Example of horizontal and vertical cone resistanceregistration (Sand 1, Dr = 0.43)

An example of the resulting cone resistance re-gistrations is given in figure 5 for a relative densityDr = 0.43. The vertical depth has been plotted withthe top of the tank as the reference level z = 0. Thehorizontal position is plotted relative to the inside ofthe tank wall x = 0. From this figure it can be seenthat at least at low densities there is no visible influ-ence of the vertical penetrations on the horizontal one.At very high densities a limited reduction of the ho-rizontal cone resistance due to the retraction of thevertical cones can be observed. This effect can easilybe corrected for, as the spatial influence of this effectis limited and the horizontal cone resistance outsidethis zone is not influenced.

For the locations of closest passage the cone resist-ance and sleeve friction have been determined. Themeasurements have been normalised by the effectivevertical stress v and are plotted as horizontal vs. ver-tical measurement in figures 6 and 7.

qHc /v

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Figure 6. Horizontal vs. vertical cone resistance

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Figure 7. Horizontal vs. vertical sleeve friction

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Sand 1Sand 2Sand 3

Figure 8. Ratio of horizontal over vertical cone resistancevs. relative density

5 HORIZONTAL CONE RESISTANCE

From figure 6 it can be seen that although there is somedispersion, on average the horizontal cone resistanceis greater than the vertical. This becomes even clearerwhen the ratio qH/Vc is plotted vs. relative density, asin figure 8. In that case the results correspond wellwith earlier findings by the authors (Broere & van Tol1998) that horizontal cone resistance is slightly lar-ger than vertical, although the observed trend that theratio qH/Vc is largest for intermediate densities is notsubstantiated by this data set.

When the separate sand types are considered thereis no clear influence on the cone resistance ratio. Theaverage of qH/Vc per sand type increases slightly from1.05 for sand type 1 to 1.1 for sand type 3, but thedispersion and errors in the data are such that this is nota significant difference. As such there is no evidenceof an influence of the gradation or coarseness of thesand on the cone resistance ratio qH/Vc .

6 HORIZONTAL SLEEVE FRICTION

In contrast to the cone resistance, the horizontal sleevefriction as well as the friction ratio show a clear influ-ence of the sand type. This can already been seen infigure 7, but becomes even more evident when the theratio of horizontal over vertical sleeve friction and theratio of horizontal over vertical friction ratio are con-sidered. See figure 9 for a plot of sleeve frictions ratiofH/Vs vs. relative density.Although the dispersion in the data is large, espe-

cially for sand 1, there is a significant decrease offH/Vs with increasing grain size d50. The mean f H/Vs

per sand type is equal to 1.28 for sand 1, 0.79 for sand2 and 0.65 for sand 3. With an average cone resist-ances ratio greater than one, this trend is even more

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Figure 9. Ratio of horizontal over vertical sleeve frictionvs. relative density

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Figure 10. Ratio of horizontal over vertical friction ratiovs. relative density

pronounced when the friction ratios are considered(RH/Vf are 1.20, 0.77 and 0.60 resp.). Given the distri-bution of the data and assuming that these ratios havea normal distribution around their means, there is aless than 1% chance that those difference are causedby a dispersion in the data. The averages for sand 1decrease somewhat when the two sleeve friction ratios> 2 are neglected, to 1.11 and 1.01, but even in thatcase the differences remain statistically significant.

This decrease in friction ratios ratio RH/Vf is causedby simultaneous changes in the separate friction ra-tios. Figure 11 shows the horizontal friction ratioplotted against the vertical friction ratio. From thisfigure it becomes clear that the relative decrease ofRH/V

f between sand 1 and 2 is caused by a decreaseof the horizontal friction ratio that is stronger than thedecrease in the average vertical friction ratio. The fur-ther decrease between sand 2 and 3 on the other handis caused by an increase of the average vertical frictionratio only.

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RHf (%)

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0 0.5 1 1.5 2RVf (%)

Sand 1Sand 2Sand 3

Figure 11. Horizontal vs. vertical friction ratio

And although the influence of the coarseness andgradation of the material on the sleeve friction is wellknown, the observed influence on the horizontal sleevefriction and especially on the horizontal friction ratioremains surprising. After all, the sands 2 and 3 exhibita clearly different friction ratio, whereas they havethe same horizontal friction ratio. If this phenomenaalso occurs for other soil types, the characterisation ofsoils based on their horizontal friction ratio becomesmore difficult and any calibration charts for verticalCPT should be used with extreme care in the detailedinterpretation of HCPT.

7 CONCLUSIONS

The cone resistance and sleeve friction measured ina horizontal cone penetration test differ from thoseobtained with vertical tests. The fact that for me-dium dense sands the average value of qc measuredin HCPT is 1.2 times the value measured in verticalCPT had been recognised already, but this observationhad been made for a fine uniformely distributed sandonly. To investigate the influence of the grain size dis-tribution on HCPT measurements new tests have beenperformed in three differently graded fine to coarsesands.

Within each sand type only a limited number of testshas been performed. These tests show no influence ofthe sand type on the ratio of horizontal over verticalcone resistance. They do show however a strong in-fluence on the ratio of horizontal over vertical sleevefriction, as the ratio fH/Vs decreases significantly withincreasing coarseness of the sand. The same is true forthe ratio of horizontal over vertical friction ratioRH/Vf .This decrease is caused by changes in the horizontalas well as the vertical friction ratio, and these changesare non-linear with the change in characteristic grainsize or distribution. No satisfactory explanation of thisdependancy on the sand type has been found.

Given the limited amount of sand types tested thequestion whether such behaviour also occurs for dif-

ferent sand types, or for different soil types in general,remains unanswered. The research shows howeverthat in detailed soil analyses based on horizontal conepenetration the correlation charts developed for ver-tical CPT should be used with care.

ACKNOWLEDGEMENTS

The authors wish to thank Mr. K.S.M.K. Kosgoda forhis help in performing the tests.

REFERENCES

Broere, W. & A.F. van Tol 1998. Horizontal cone penetra-tion testing. In Robertson, P.K. & P.W. Mayne (eds),Geotehnical Site Characterization, Proc. ISC98, pp.989994. Balkema.

Deen, J.K. van, G. Greeuw, R. van den Hondel, M.Th. vanStaveren, F.J.M. Hoefsloot & B. Vanhout 1999. Hori-zonal CPTs for reconnaissance before the TBM front. InBarends, F.B.J., J. Lindenberg, H.J. Luger, L. de Quelerij& A. Verruijt (eds), XII. ECSMGE Geotechnical Engin-eering for Transportation Infrastructure, pp. 20232030.Rotterdam, Balkema.

Houlsby, G.T. & R. Hitchman 1988. Calibration cham-ber tests of a cone penetrometer in sand. Gotechnique,38(1):3944.

Jamiolkowski, M., V.N. Ghionna, R. Lancelotta &E. Pasqualini 1988. New correlations of penetration testsfor design practice. In Ruiter, J. de (ed.), PenetrationTesting 1988, pp. 263296.

Lunne, T., P.K. Robertson & J.J.M. Powell 1997. ConePenetration Testing in Geotechnical Practice. London,Blackie.

Parkin, A.K. 1988. The calibration of cone penetrometers.In Ruiter, J. de (ed.), Penetration Testing 1988, pp. 221243.

Ruiter, J. de (ed.) 1988. Penetration Testing 1988, Rotter-dam. Balkema.

Salgado, R., J.K. Mitchell & M. Jamiolkowski 1998. Cal-ibration chamber size effects on penetration resistance insand. ASCE Journal of Geotechnical and Geoenviron-mental Engineering, 124:878888.

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IntroductionHCPT experiencesReseach aims

The calibration chamberThe sandsOverview of test seriesHorizontal cone resistanceHorizontal sleeve frictionConclusions

Documents

Horizontal Cone Penetration Testing in Sand